refactor: 移除备份恢复功能(API/CLI/手册)

日志数据的保留与安全由对象存储 + Loki retention 承担,平台侧整库备份恢复价值有限:
删除 services/backup.go、/system/{backup,restore,restore/apply,backups} 路由与 handler、
CLI backup/restore 子命令、前端 api 方法与类型;自检所需的 dataDirPath/密钥指纹并入 selfcheck;
用户手册删备份章节并修正 FAQ 升级建议;CHANGELOG 记录移除。

backend/routers/router.go

@@ -147,10 +147,6 @@ func SetupRouter() *gin.Engine {
 	system := authorized.Group("/system")
 	system.Use(middleware.AdminAuth())
 	{
-		system.GET("/backup", systemCtrl.Backup)
-		system.POST("/restore", systemCtrl.Restore)
-		system.POST("/restore/apply", systemCtrl.RestoreApply)
-		system.GET("/backups", systemCtrl.Backups)
 		system.GET("/self-check", systemCtrl.Health)
 		system.GET("/metrics-summary", middleware.AdminAuth(), systemCtrl.MetricsSummary)
 	}